In wireless communication systems, wireless service providers may operate radio access networks (RANs), each RAN including a number of base stations radiating to provide coverage in which to serve user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices. In turn, each base station may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the RAN may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other UEs served by the base station.
Further, a RAN may operate in accordance with a particular air interface protocol or “radio access technology,” with communications from the base stations to UEs defining a downlink or forward link and communications from the UEs to the base stations defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Orthogonal Frequency Division Multiple Access (OFDMA (e.g., Long Term Evolution (LTE) and Wireless Interoperability for Microwave Access (WiMAX)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), and Global System for Mobile Communications (GSM), among others. Each protocol may define its own procedures for registration of UEs, initiation of communications, handover between coverage areas, and other functions related to air interface communication.
In general, a wireless service provider may operate one or more such RANs as a public land mobile network (PLMN) for serving UEs that subscribe to service of the provider. For example, a service provider may operate a CDMA PLMN and may provide UEs with subscriptions that allow the UEs to receive CDMA service from that PLMN. As another example, a service provider may operate an LTE RAN as a PLMN and may provide UEs with subscriptions that allow the UEs to receive LTE service from that PLMN. And as another example, a service provider may operate both a CDMA PLMN and an LTE PLMN and may provide UEs with subscriptions that allow the UEs to receive both CDMA service from the CDMA PLMN and LTE service from the LTE PLMN.
A RAN operating as a PLMN may have an associated PLMN identifier (PLMN ID), and base stations of the RAN may be arranged to broadcast that PLMN ID to indicate that the base stations are part of that PLMN. UEs that subscribe to service of a wireless service provider's PLMN may then be provisioned with data indicating the PLMN ID of the PLMN and with logic that causes the UEs to prefer service by base stations broadcasting that PLMN ID. Further, UEs that subscribe to service of multiple PLMNs, such as both an LTE PLMN and a CDMA PLMN may be provisioned with data indicating the PLMN IDs of each such PLMN and with logic that causes the UEs to prefer service by base stations broadcasting one or more of those PLMN IDs.
Still further, a wireless service provider may allow one or more of its PLMNs to serve UEs that subscribe to service of other PLMNs, pursuant to a roaming agreement. In particular, a first service provider providing a first PLMN may enter into a roaming agreement with a second service provider providing a second PLMN, according to which the first PLMN will serve UEs that subscribe to the second PLMN, and the second service provider will compensate the first service provider for providing that service. As such, a UE that subscribes to service of the second PLMN but that is not within sufficient coverage of the second PLMN may instead opt to be served by the first PLMN.
In practice, a base station may be configured to provide service on multiple carrier frequencies or “carriers.” In particular, the air interface between the base station and served UEs on both the downlink and the uplink may span a particular frequency bandwidth (such as 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, or 20 MHz) that may be divided primarily into subcarriers (e.g., subcarriers spaced apart from each other by 15 kHz). Further, the base station may support one or more frequency bands, such as the 800 MHz band (one or more frequency ranges around 800 MHz), the 1.9 GHz band (one or more frequency ranges around 1.9 GHz), and the 2.5 GHz band (one or more frequency ranges around 2.5 GHz), and may provide service on one or more carriers that reside in each supported band. In a frequency division duplex (FDD) arrangement, different carriers are used for the downlink than the uplink. Whereas, in a time division duplex (TDD) arrangement, the same carrier is used for the downlink and uplink and is allocated over time among downlink and uplink communications.
Each carrier may define various logical channels to facilitate communication between the base station and one or more served UEs. For instance, on the downlink, a carrier may define a reference channel on which the base station broadcasts a reference signal useable by UEs to detect and evaluate coverage, various other downlink control channels to carry control signaling (such as resource-scheduling directives) to UEs, and one or more shared or traffic channels for carrying bearer data (e.g., user or application level data) to UEs. And on the uplink, a carrier may define one or more uplink control channels to carry control signaling (such as resource scheduling requests, channel state reports, and the like) from UEs, and one or more shared or traffic channels for carrying bearer data from UEs.
When a UE enters into coverage of a base station on a particular carrier, the UE may attach or register with the base station on that carrier, and the base station may then serve the UE on that carrier. Further, under certain air interface protocols, a base station may be able to serve a UE concurrently on multiple carriers, to help increase the effective bandwidth and associated throughput available to the UE. For instance, if a UE is attached with a base station on a first carrier, the base station may then add a second carrier to its service of the UE so as to then provide the UE with “carrier aggregation” service on a combination of the first carrier and the second carrier. In that arrangement, the first carrier may be considered the UE's primary carrier or primary cell (PCell), and the second carrier may be considered the UE's secondary carrier or secondary cell (SCell). Depending on the carrier aggregation implementation, the SCell might be used principally for downlink communication (to increase the UE's downlink throughput) rather than for uplink communication, and the PCell may carry some or all control signaling related to the SCell (in addition to control signaling related to the PCell).